In conventional optical communication, a direct detection scheme that utilizes ON/OFF of a light intensity to perform communication is generally used. However, with the recent spread of the Internet, a communication scheme having a larger transmission capacity is demanded in a backbone optical communication system in particular, and a digital coherent reception scheme having a combination of an optical coherent reception scheme with a higher reception sensitivity than the direct detection scheme and a digital signal processing technology has gained attention (see, e.g., Liu, Xiang, “DSP-Enhanced Differential Direct-Detection for DQPSK and m-ary DPSK”, European Conference on Optical Communication (ECOC) 2007, 7.2.1; and Ly-Gagnon, Dany-Sebastien; Tsukamoto, Satoshi; Katoh, Kazuhiro; and Kikuchi, Kazuro, Member, IEEE, Member, OSA, “Coherent Detection of Optical Quadrature Phase-Shift Keying Signals With Carrier Phase Estimation”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 24, No. 1, January 2006, pp. 12-21).
The digital coherent reception scheme is a scheme that combines a received optical signal with reference light to convert intensity and phase information of the received optical signal into an extractable signal, converts this signal into a digital signal by an analog/digital converter (ADC), extracts the intensity and phase information of the optical signal from the converted digital signal, and performs digital signal processing with respect to the extracted signal, thereby demodulating the received signal. As the reference light, a received signal delayed for one symbol time may be used, or light (local light) output from another laser provided on a reception side may be used.
Digital coherent reception is characterized in that highly accurate phase control of the reference light is not required as compared with a conventional coherent reception scheme because digital signal processing technology is used to compensate phase difference of the received signal and the reference light without performing optical phase synchronization of the received optical signal and the reference light. Further, highly accurate waveform distortion compensation can be performed by an electrical equalizing filter since information indicative of both amplitude and phase of an optical electric field of the received optical signal can be acquired as an electrical signal.
As a modulation scheme when using digital coherent light reception, not only a binary modulation scheme as typified by intensity modulation but also multi-ary phase shift keying (MPSK), e.g., differential quadrature phase shift keying (DQPSK) or quadrature amplitude modulation (QAM) can be realized by the same receiver configuration.
As a general method for recovering a clock in a digital signal processing circuit, two methods can be considered. When a clock recovery unit is provided on an upstream side of a waveform distortion compensator in the digital signal processing circuit, because a signal on the upstream side of the waveform distortion compensator has a distorted waveform, occasionally a sampling clock signal cannot be recovered when waveform distortion is severe. Thus, the ADC cannot perform digital conversion.
Even if the clock signal can be recovered, since the quality of the recovered clock signal is poor, accuracy of digital conversion in the ADC is reduced. Conversely, provision of the clock recovery unit on a downstream side of the waveform distortion compensator of the digital signal processing circuit can be considered; however, loop delay of the recovered clock signal to the ADC increases. Hence, there is a problem in that the quality of the clock signal is poor and accuracy of digital conversion is reduced.